Thyroid-specific enhancer-binding protein (T/EBP) is a homeodomain- containing DNA-binding protein which controls expression of thyroid- and lung-specific genes such as those encoding thyroid peroxidase, thyroglobulin, and the thyroid stimulating hormone (TSH) receptor, and surfactant proteins A, B, and C, and clara cell secretory protein, respectively. A role for T/EBP in mammalian development has been suggested after T/EBP was found to be expressed in rat embryos. Our earlier work on targeted disruption of the mouse T/ebp gene has conclusively demonstrated that T/EBP is essential for organogenesis of the thyroid, lung, ventral forebrain, and pituitary. In order to understand how T/EBP gene expression is controlled and which of its target genes are involved in embryogenesis and in maintenance of the differentiated function of the thyroid and lung, we characterized the upstream sequence of the rat and human T/EBP genes by using cDNA and genomic clonings, northern blotting, and transfection assays. The T/EBP gene possesses a novel structure in the 5'non-coding region that results from differential transcription initiation and splicing. The T/EBP cDNAs can be grouped into 6 different types; four contain an intron of different length whereas the other two contain no intron. Longer T/EBP mRNAs are more abundant in rat FRTL-5 cells maintained in the absence of TSH as compared to its presence, suggesting a correlation between premature mRNA utilization and the physiological state of the cells where T/EBP is expressed. The T/EBP gene expression was further found to be auto-regulated. Detailed analyses of mechanisms controlling T/EBP gene expression are currently under study.